It is known that liquid condensates constituting the byproducts of gas production (natural gas, associated gas) and crude oil can contain numerous metallic compounds in the trace state and in particular mercury.
These charges are increasingly steam cracked and the consequence of this is to produce effluents polluted by mercury. The catalysts of the transformation processes located downstream of the steam cracking process are consequently severely poisoned, which in certain cases makes the gas condensates economically less attractive. In addition, mercury may also cause corrosion through the formation of amalgams with aluminium-based alloys, particularly in sections of the plant operating at a sufficiently low temperature to cause the condensation of liquid mercury (cryogenic fractionation, exchangers).
The prior art discloses numerous products having an interesting demercurizing activity. Thus, there are masses constituted by active carbon impregnated with sulphur or iodine in U.S. Pat. No. 3,194,629 and copper oxide in U.S. Pat. No. 4,877,920. U.S. Pat. No. 4,843,102 claims the use of a polysulphide-impregnated resin, as does U.S. Pat. No. 4,877,515, which deposits these polysulphides on a molecular sieve. According to U.S. Pat. Nos. 4,892,567 and 4,874,525 sometimes the molecular sieve is impregnated by a metal known to form amalgams such as gold or silver. U.S. Pat. No. 3,194,629, assigned to Institut Francais du Petrole (IFP) describes other masses incorporating copper at least partly in the sulphide form and a mineral support.
These different masses are very effective for trapping the metallic mercury, but relatively ineffective when the metal is combined in the form of dimethyl mercury or mercury compounds having hydrocarbon chains containing two or more carbon atoms and optionally other non-metallic elements (sulphur, nitrogen, etc.). In order to ensure a complete elimination of the mercury in all cases, IFP has described two processes constituted by two reaction zones in series containing two masses preferably used in series, the first carrying out the hydrogenolysis of the organometallic compounds in the presence of hydrogen and the second trapping most of the mercury contained.
Thus, French patent application FR-A-89/03581 describes the first catalyst as containing at least one metal M from the group constituted by nickel, cobalt, iron and palladium and followed by or mixed with a trapping mass containing sulphur or a metal sulphide.
French patent application FR-A 89/03500 describes the first catalyst as containing at least one metal M chosen from the group formed by nickel, cobalt, iron, palladium and platinum, at least one metal N chosen from the group formed by chromium, molybdenum, tungsten and uranium followed by or mixed with a mercury trapping mass containing a sulphide of at least one metal P chosen from the group formed by copper, iron and silver, or sulphur.
As these two processes operate in the presence of hydrogen, they cannot be used on cracked gases, which contain significant quantities of unsaturated compounds. It is therefore necessary to use them upstream of the steam cracker, while pretreating the charge. Such a situation is very interesting, because it makes it possible to ensure the elimination of the mercury from the entire installation on all the unsaturated fractions produced. It also prevents mercury pollution of installations in contact with the cracked gases prior to their separation into different fractions.
Nevertheless many steam cracking installations (zones) have operated with mercury-polluted charges without using any mercury elimination technology at all. In this case the complete installation is polluted by mercury and the limited, but not negligible, steam affinity for of this metal means that even the mercury-free effluents, because the steam cracker charge has been treated, could again become charged with mercury.
It is also standard practice to "import" unsaturated fractions in addition to those produced in situ in order to increase the flexibility of the installation. In this case it is not sufficient to eliminate the mercury from the steam cracker charge and it is also necessary to install trapping masses on the cracked products. In this case it is possible to use the various products referred to hereinbefore, but they all suffer from the disadvantage of a polluting reprocessing.
Thus, resins must be treated with solutions which entrain the trapped mercury, which imposes expensive downstream treatments. Carbons must be burned in order that it is possible to recover the active phase deposited on their surface, which causes a problem of serious pollution of the combustion fumes, which are seriously polluted by the volatile mercury. Molecular sieves are usually regeneratable, but here again it is necessary to have an installation making it possible to eliminate the mercury from the regeneration effluents.
However, the mercury trapping masses used upstream, like those described in French Application FR-A-89/03581 and 89/03500 are mineral products which have a composition similar to the ores containing the mercury. It is therefore standard practice to treat such solids in order to extract therefrom the metallic components and also the mercury contained.